Occult biochemical pregnancy: fact or fiction?

Daily urine samples were obtained from 38 women of proven fertility, who were attempting to conceive, to assess the frequency of clinically unsuspected "biochemical pregnancies' on the basis of late luteal phase rises of hCG. In 25 clinically confirmed conception cycles, the earliest pregnancy-induced hCG rises from baseline occurred 8 days after the endocrinologically defined date of ovulation (mean 11.1 days, SD 1.3). Urine samples obtained throughout 50 non-conception ovulatory cycles provided no evidence of any 'biochemical pregnancies' on the basis of late luteal phase rises of hCG occurring 8 or more days after the endocrinologically defined date of ovulation. If, as in previous studies, the date of ovulation had been estimated on the basis of menstrual cycle length and only the relatively non-specific NIH antiserum had been used in RIAs, 7 of the 50 (14%) cycles would have been classified as resulting in biochemical pregnancies on the basis of hCG greater than 50 m-i.u./ml on one occasion or greater than 20 m-i.u./ml on two or more occasions after day 20 of the cycle. These levels of 'hCG' occurred at or soon after ovulation and could be explained by LH cross-reacting in the RIA. The rises of 'hCG' were not confirmed when the urine samples were retested with an antiserum (Wellcome 895) of very high specificity for hCG. These results raise the possibility that the frequency of clinically unsuspected 'biochemical pregnancy' may have been substantially overestimated.